A sign Endeavour isn't going anywhere. The rotating service structure is rolled back around the shuttle at pad 39A on Tuesday. Photo: NASA

While workers make plans to replace a faulty avionics box aboard shuttle Endeavour, NASA officials are jockeying to secure a new launch date for the Earth radar mapping mission.

The U.S. space agency says Endeavour will be ready to lift off on February 9, but an unmanned rocket launch and countdown rehearsal for another expendable booster at nearby Cape Canaveral could delay the shuttle until February 12.

NASA cancelled Tuesday's launch attempt of Endeavour prior to fueling the shuttle. Managers met before dawn and decided one of two Enhanced Master Events Controllers aboard the ship needed to be replaced. The units send the vital commands to ignite the twin solid rocket boosters at liftoff, and later separate the spent boosters and external tank during launch.

The suspect EMEC No. 2 exhibited unexplained behavior during Monday's launch attempt that was thwarted by bad weather.

Engineers troubleshooting the EMEC say it failed a standard pre-flight test at T-minus 29 minutes, but later responded properly during follow up examinations. Further analysis and testing indicated a possible hardware problem inside the unit.

The EMEC, located in the shuttle's aft engine compartment, weighs 65 pounds, is about 20 inches long, 13 inches wide and 8 inches tall.

A spare unit that will be installed into Endeavour was tested at Kennedy Space Center on Tuesday.

Here is the game plan over the next few days:

• On Wednesday, workers plan to drain the liquid oxygen and liquid hydrogen from the shuttle's three fuel cells and disconnect ordnance lines. Once those chores are completed, efforts to remove EMEC No. 2 will begin, probably late Wednesday.
• Installation of the new unit and subsequent retesting should be completed by late Thursday night.
• On Friday, workers will reconnect the ordnance. Aft compartment close-outs will conclude over the weekend.

If all goes well, NASA would be ready to start a new three-day countdown on Sunday morning, leading to liftoff on Wednesday, February 9.

However, that launch date is not an option currently.

A U.S. Air Force Titan 4B rocket sits atop its Cape launch pad. Photo: USAF

The U.S. Air Force-controlled Range, which provides the required tracking, communications and safety services for all launches from KSC and neighboring Cape Canaveral Air Station, is unavailable to support Endeavour until Saturday, February 12.

The Range is facing a log-jam of activity next week. Complicating matters is the 48 hours it takes to reconfigure the Range's systems from one launch vehicle to another.

First up is the planned February 8 launch of a Boeing Delta 2 rocket from Cape Canaveral carrying four Globalstar cellular telephone satellites. Boeing would like to have a backup launch date on the 9th in case of a delay.

But that is not currently possible because of the already-scheduled countdown rehearsal for an Air Force Titan 4B rocket at the Cape. The Titan test is planned for the 10th to check out the rocket's systems prior to its launch later in the month. Since the test needs Range support, the 9th and 11th will be spent switching Range equipment from the Delta to the Titan, then from Titan to shuttle, respectively.

The bottom line is Endeavour, although it could be ready to fly on February 9, won't get a chance to launch until February 12. Negotiations are continuing between all the parties and things could change, so stay tuned.

As for Endeavour's six astronauts, they will leave Kennedy Space Center at about 1500 GMT (10 a.m. EST) on Wednesday and return to their homes in Houston. The crew will return to KSC about four days prior to the next launch attempt.